This invention relates to backflow prevention devices and, particularly, to improved check valves of the spring loaded poppet type for use in a backflow preventer.
Backflow preventers utilizing check valves of the spring loaded poppet type for preventing contamination of water supplies as a result of back flow or pressure reversal in water supply lines have been known in the art. However, recent developments in this field have necessitated redesign efforts in regard to the construction of the check valve portion of the devices to improve the operational characteristics thereof. Specifically, industry and governmental standards for backflow prevention devices have been modified so that these devices must now achieve lower pressure loss at rated flow and/or higher sealing pressures at no flow than prior devices in order to meet the standards.
It has been found that these objectives can be accomplished in backflow prevention devices utilizing spring loaded poppet type check valves by internal streamlining of the valve body and, more importantly, by employing specially designed springs. These special springs optimally have buckling ratios as determined by dividing the free length of the spring by its pitch diameter (L/D) approaching 4.00 and use of such springs enables the designer to select an optimal spring pitch diameter as well as the lowest rate spring (i.e., ratio of spring force to spring deflection) suitable for such a pitch diameter. The "pitch diameter" used herein is also referred to elsewhere as the mean diameter of the spring helix or simply the mean diameter and may be determined by measuring the outside diameter of the spring perpendicular to the center line axis of the spring and subtracting the wire diameter from the measurement. The selected low rate spring is important to provide increased closing forces within the valve body without adversely affecting pressure loss at rated flow. However, in operation, fluid flow within the valve housing has been found to induce serious vibrational movement of these springs having buckling ratios of about 4.00 which causes amplification of flow disturbances within the housing. The flow disturbances in turn cause excessive pressure drops at rated flow. In addition to the undesirable pressure drops at rated flow, it has also been found that objectionable clattering noises are encountered in the valves as a result of spring surging of the higher buckling ratio springs. This disadvantageous noise factor occurring from collision of adjacent turns of the higher buckling ratio springs as fluid in the valve housing flows across the springs is present throughout a wide range of fluid flow rates.
These and other disadvantages relating to the amplified disturbances resulting from flow excited spring vibrations and the clattering noise from spring surging are overcome by the present invention wherein there is provided a shield to guide fluid flow around the outside of the spring rather than allowing flow through the spring windings.